1. Field of the Invention
The invention relates in general to a local oscillation generator for a communication system, and more particularly, to a local oscillation generator capable of preventing an amplifier from interfering with a local oscillation circuit in a communication system.
2. Description of the Related Art
Communication networks of various standards including WLAN, Bluetooth, mobile communications, satellite positioning, and digital televisions constitute an essential part of the modern information society. A communication network includes a transmitter and a receiver. To fully utilize communication channels as well as perform multiple accessing, the transmitter and the receiver respectively includes a local oscillation generator for generating local oscillation signals. For example, in a radio-frequency (RF) wireless communication network, a transmitter encodes information to be transmitted to the communication network to a baseband signal, and a local oscillation signal provided by a local oscillation generator at the transmitter serves as a modulation carrier. By mixing the baseband signal and the modulation carrier, the baseband is modulated and up-converted to an RF signal. A power amplifier at the transmitter further amplifies the power of the RF signal in order to transmit the RF signal to a communication network medium, e.g., the air. Correspondingly, after a receiver in the RF communication network receives the RF signal via the network communication medium, a local oscillation signal provided by a local oscillation generator serves as a demodulation carrier. By mixing the RF signal and the demodulation carrier, the RF signal is demodulated and down-converted to a baseband signal to restore the original information from the transmitter.
In a conventional local oscillation generator, an original oscillation signal is generated by an oscillation circuit, and the original frequency signal is frequency divided by an integral multiple to generate a local oscillation signal. For example, assuming that the local oscillation frequency required by the local oscillation signal is 2.4 GHz, a 4.8 GHz or 9.6 GHz original oscillation signal is first generated by an oscillation circuit of the conventional technique, and is then frequency divided by an integral multiple of 2 or 4 to obtain a 2.4 GHz local oscillation signal. Moreover, due to non-linearity of circuits in the transmitter or circuits in the receiver, the local oscillation signal induces harmonic multi-frequency oscillation signals containing frequencies in integral multiples of the local oscillation signal, e.g., 4.8 GHz or 9.6 GHz. However, in the above conventional technique, since the original oscillation frequency based on which the oscillation circuit operates is also an integral multiple of the local oscillation frequency, the harmonic multi-frequency oscillation signals interfere with the operations of the oscillation circuit to degrade the communication quality.
For example, in the transmitter, as the power amplifier amplifies the power of the RF signal, the non-linearity of the amplification process induces high-power harmonic multi-frequency oscillation signals. In the event that the original oscillation frequency of the oscillation circuit for providing the original oscillation signal is also an integral multiple of the local oscillation signal, the harmonic multi-frequency oscillation signals of the power amplifier imposes a frequency pulling effect on the oscillation circuit. As a result, an error vector magnitude of the transmitter is deteriorated to degrade the overall signal quality. Such an effect is quite severe in various 2G/3G/4G mobile communications networks and WLAN. A main reason for the above is that, in a transmitter of the above wireless networks, a power amplifier operates at a higher power such as 15 to 30 dBm (or 32 to 1000 mW).